Author contributions

Author contributions: C.F., F.Y., and S.W. designed research; C.F., L.S., Z.W., H.Y., Y.H., Y.L., and F.Y. performed research; C.F., L.S., H.Y., Y.T., F.Y., and S.W. analyzed data; Y.Z., X.Z., and F.Y. edited the paper; S.W. wrote the first draft of the paper; S.W. wrote the paper.

Disclosures

The authors declare no competing financial interests.

This work was supported by grants 31371166 and 31571174 to S.W. from National Natural Science Foundation of China and grant C2014206303 from Hebei Natural Science Foundation, and grant CXZZBS2017102 to C.F. from Hebei Province Government. We are grateful to Professor Sergey Kasparov for his comments on an earlier version of the manuscript.